The Fine Art of Digital Printing: PPI? DPI? — Part 2

Last time I talked about PPI (pixels per inch) — click here to read the first article. This time it’s DPI.  People use the terms interchangeably, but they aren’t the same. In fact, DPI doesn’t even mean just one thing.

Figure 1: This photograph prints out nice and sharp at 11x14" image size.

DPI originally stood for “dots per inch.” That meant the halftone dots that make up a picture on a printed page, like in a newspaper or magazine. (Remember those?)  Since each halftone dot can render a variety of gray tones from black to white, depending on how large or small it is, you would think a “dot” would be the equivalent of an “pixel.” It doesn’t work out that way. The software that can turn digital images into halftone screens generally wants more pixels than the number of dots. How many more seems to depend upon the software and the skill of the printer, but for a high quality 165 DPI magazine illustration, you frequently want something between a 240 and a 300 PPI image (of the same physical size in inches) for good quality reproduction.

Figure 2: A very small section of the print, highly enlarged, shows the actual inkjet ink droplets. They are much, much smaller than the finest detail in the photograph.

You think that’s confused? Inkjet printers, which are likely to be what you’re making your fine prints on, also are specced in “DPI,” but now we’re talking about “(ink) Droplets Per Inch!”  A high-quality printer that’s advertised as offering, say, 2880 x 1440 DPI  can lay down ink droplets at a spacing of 2880 per inch in one direction and 1440 per inch in the other direction.  But, each ink droplet can only portray one (or a few) levels of tone. To get what we think of as continuous tone with 256 or more levels per color you need a whole spray of inkjet drops; the closer together they are, the darker the color appears. The actual resolution of the printer in terms of image detail is very much lower than its DPI rating, because it takes a lot of inkjet droplets to build up one photographic pixel.

As I said, confusing!

So, how many PPI do you need in a file to get a nice, sharp print from one of today’s high-quality inkjet printers? Well, that will be the topic of the next two columns.

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